This invention relates to an improved chroma keying system and to an improved circuitry for use therewith that can be continuously adjusted to select any color of interest from a video source.
Chroma keying systems are available commercially in broadcast equipment. These chroma keying systems are used where it is desired to place a second video signal in the background of a first video signal. For example, during a TV news broadcast it may be desirable to display a news event from a second camera in the background of the news reporter in front of a first camera. This is done with a chroma keyer by having a background of a particular color, such as blue, for example, which is not generally found in skin tones. The chroma keyer would be responsive to signals picked up from the first color camera having this blue color for switching in the signal from the second camera or video source that contains the news event to be displayed.
It is desirable that this chroma keying system be as selective as possible in order that the system not key on adjacent colors. For example, some reporters may have blue eyes which are so closely related in the system to the chroma keying color in the background that the reporter's eyes would be blanked and the scene from the second camera appear at the eyes. It is also desirable that this selective control be an adjustable control where any color may be selected from the video source so as to establish any keying signal and further it is highly desirable to provide a remote control of this keying signal. It is desirable that a control of this chroma keying be remote and not at the camera location so that the person controlling the mixing of the two or more cameras would be able to select the keying signal.
A chroma keying system with remote control with adjustable color is described in U.S. Pat. No. 3,560,638 of Skrydstrup et al. In this system, a filter produces a signal having the form (R-Y) sin .theta.+(B-Y) cos .theta., where .theta. is a function of control voltages applied to voltage controlled amplifiers in the filter. The filter acts to provide a maximum transfer (lowest loss) at a desired color selected by the control voltages. The switching to the second camera occurs only when the signal level from the filter exceeds a given threshold level which threshold level is overcome at the particular desired color passed by the filter. While the arrangement in the patent described above may provide a threshold voltage for certain selected colors with little difficulty, this system does not have the sharp rise time characteristic desired to reject nearby colors. In an effort to overcome this problem, chroma keying arrangements have been developed using the red, green and blue difference signals and non-additive mixers. Switching circuits using diodes for example have been used to combine color difference signals to the non-additive mixer to provide a number of pre-selected color signals according to the diode biasing. It is desirable to provide a system which is variable between these preselected colors remotely where these output signals would have the sharp rise time characteristics necessary to reject the nearby colors.